There are tough-to-fathom omissions like leaving off a reception for AIDS activists and researchers that had already been announced on a White House blog. And seemingly deliberate ones, such as the absence of several meetings with House Speaker John Boehner at the height of debt ceiling negotiations. Others seem designed to downplay inconvenient or politically awkward events, such as meetings about the Sudan with actor George Clooney, a major contributor and fundraiser for the president.

A review of more than 4,000 photos posted to Flickr by official White House photographers since Obama took office in 2009 found about one-third of the events depicted were never on his public schedule. And, of course, many other unannounced events never make it to the Flickr feed at all.

The gaps are a reminder of the limits of transparency, even for a White House that makes it a priority. There is no legal requirement to put out a White House schedule. The president’s complete daily schedule can’t be obtained directly under the Freedom of Information Act, since the White House is exempt from that law.

Presidents also fiercely guard their prerogatives to see — and not see — whomever they want. So the public is largely at the mercy of Obama and his staff to put out the information they choose and, as the POLITICO review shows, shape it in ways big and small to suit their needs.

The events shown in the photos but omitted from the schedule range from meetings with congressional leaders to foreign and domestic policy briefings to sessions with sick children and veterans. Obama’s interactions with Democratic constituency groups and advocates for causes — visits often characterized as presidential “drop-bys” — are frequently absent from the schedule but pop up in the Flickr feed.

No one would expect every phone call or casual encounter to appear on a public schedule, and of course, certain national-security related events would be left off. But sometimes it appeared the White House was trying to showcase events to select audiences while not calling the broader White House press corps’ attention to aspects of the president’s schedule that might undercut the relentless “jobs, jobs, jobs” focus Obama aides seem eager to promote.

Based on the Flickr feed and news reports, here are a few notable meetings that never made Obama’s public schedule:

Obama’s public schedule is posted daily on the White House website. The truly obsessive can even have it automatically added to their iPhone or BlackBerry calendars.

But, especially when Obama is at the White House, long stretches of time are often unaccounted for.

Reporters who cover Obama’s White House regularly learn of the president’s meetings through means other than the daily schedule, such as tweets from guests or blog entries that White House aides post later.

“Too often President Obama has meetings with foreign officials, members of Congress or representatives of interest groups that never make it to the schedule given to the press the night before,” said Mark Knoller, a longtime White House correspondent for CBS News. “Sometimes we only find out about the events from non-White House sources: embassies, interest groups [and] congressional offices.”

“I would say the daily schedule they hand out is on the shallow side compared to presidents past,” said another White House veteran, Ann Compton of ABC News.

The White House says it gives the public and the press unprecedented insight into the president’s daily activities through the public schedule and a variety of other sources.

“Through the daily guidance, an active blog (at whitehouse.gov), a revealing, behind-the-scenes Flickr site and a responsive communications staff, the President has met his goal of having the most transparent White House in history — while protecting his ability to appropriately handle the many sensitive topics that come across his desk every single day,” White House deputy press secretary Josh Earnest said in a statement.

After Obama faced criticism for passing up an invitation to speak at a United Nations-sponsored AIDS conference in Washington this summer, the White House said in a blog post it would host a reception for conference delegates. The event wasn’t on Obama’s schedule, but senior adviser Valerie Jarrett revealed his attendance on Twitter. “Today I joined President Obama @ reception honoring #AIDS conference,” she wrote.

Photos of the AIDS reception appeared on the Flickr feed.

Though the press was excluded from the event, quotes from Obama’s speech appeared in a press release issued by the United Nations Program on HIV/AIDS. And at least one of the guests posted fuzzy video to YouTube.

When Obama sat down with big-name Wall Street donors in the White House’s Blue Room on a Monday night in March 2011, reporters were in the dark about the session because it did not appear on the president’s public schedule. Word of it emerged three months later in The New York Times.

The traveling press secretary for Obama’s campaign, Jen Psaki, said last month that the president has a “regular weekly political meeting with staff.” Such events never appear on the schedules released by the White House.

An unannounced stop Obama made at a Portland, Ore., coffee shop in July led to suggestions in conservative blogs that the Obama campaign hoodwinked reporters into believing that the president just happened to find himself in conversation with three veterans there. However, the pool report on the visit makes clear that the campaign billed the lunch as a “roundtable discussion,” albeit one that came as a surprise to the press corps.

Security is often a factor in leaving some of Obama’s jaunts off the schedule, White House aides say. If his visit to a restaurant or a sporting event is announced in advance, the Secret Service applies stricter rules that can be disruptive and expensive.

A few Cabinet members — Secretary of State Hillary Clinton, Defense Secretary Leon Panetta and Treasury Secretary Timothy Geithner — show up with regularity on the public schedule. Others don’t.

When a meeting with Attorney General Eric Holder popped up on Obama’s public schedule in April 2011, reporters were so surprised that Holder’s spokesman was deluged with questions about whether the attorney general was about to resign.

The White House’s reasons for omissions on Obama’s public schedule can vary. Some, involving national security and key personnel decisions, are pretty straightforward. Few would argue the public had a right to know in advance about the president’s meetings before the raid that killed Osama bin Laden.

Other, more debatable reasons range from a desire to shape public perceptions about the president’s agenda to attempts to avoid demands for access from the public and press corps, according to former White House and administration officials.

In an interview last month, Obama seemed to acknowledge that perceptions about his schedule could hurt him politically. Clooney, who has come to the White House to talk about Darfur, is “sensitive to the fact that you know that if he’s around a lot then somehow it’ll be tagged as Obama hanging out with Hollywood stars,” the president told “Entertainment Tonight.”

White House aides say that, despite the administration’s transparency pledges, the president is entitled to meet with his official and political advisers without disclosing such sessions to the press.

One former administration official said being specific about policy meetings would simply intensify efforts by outsiders — and even insiders — to weigh in.

“The real problem is lobbyists go to town on that stuff. They’ll burn up the phone lines the moment it’s reported the president is having a meeting like that. … Not that you can’t withstand that, [and] smart ones are going to know about the stuff anyways, but I just think it makes the decision-making process more complicated,” said the former aide, who asked not to be named discussing internal strategy. “You don’t want some sub-Cabinet level person complaining they weren’t in the loop.”

Many of the omissions from the schedule, such as the AIDS conference reception, involve appearances the White House considers “drop-bys.”

White House officials don’t dispute that it’s often nearly certain that the president will attend such events. However, they say designating them as “drop-bys” and leaving them off the public schedule avoids offending attendees if the president spends only a few minutes at a briefing that lasts for hours. If the events were announced as meetings with the president, aides say, guests might feel he was committed to attend the whole session.

Putting a reception or social event on the schedule can also lead to unwelcome pleas for invitations. In April 2009, the White House accidentally sent reporters an internal email exchange about whether a Seder the president was hosting for Passover should appear on the daily schedule.

Director of Scheduling Danielle Crutchfield asked press staffers to take the Seder off the schedule sent to reporters. “Apparently Jewish [sic] here and in neighboring states are now calling wondering why they have not been invited,” Crutchfield wrote.

In the email chain, Earnest said removing the entry for the Seder was pointless, because it already had been mentioned to the press corps the day before. “Folks already know that it’s happening. They’ll wonder why it’s not listed if we don’t include it on there,” he wrote. The closed-press event remained on the public schedule.

Some journalists see the skimpy schedule as an effort to make an end-run around the press corps. While a small number of the official photos are released immediately, the vast majority emerge days or weeks later, long after most in the media have lost interest. The White House is now capable of videotaping, tweeting and blogging about events in a way that lets it manage the information and sometimes renders the traditional press irrelevant.

“It’s all about expectations. If we knew the U.N. Human Rights Commission was coming for an informal meeting with the president, people would be burning up the phone lines,” Compton said. “If we find out afterward, the White House can much better control it. … You cannot fault a political West Wing for wanting to control the agenda. God knows, there’s a lot they can’t control.”

In some instances, leaving events off the schedule seems to be part of a White House strategy to serve and satisfy various Democratic constituencies while emphasizing to the general public and the mainstream press that the president is focused on the economy.

The Flickr photos show Obama regularly makes time to meet with sick children in the Make-a-Wish program as well as wounded warriors from the armed forces . Those events are left off the public schedule to avoid perceptions that the president is trying to exploit those involved, though the photos are released publicly if the guests consent, White House aides said.

In tallying the events depicted in the Flickr photos, POLITICO excluded purely personal events such as Obama’s private interactions with his children. White House photos that don’t show Obama were not counted, except where the caption linked the image to an event he attended. In addition, photos of Obama speaking on the phone were usually excluded, except when part of a larger meeting.

But there was a degree of subjectivity to the analysis. Photos of Obama consulting informally with one or two White House advisers were often not counted. Oval Office sit-downs with a larger group of advisers were considered to be appointments that could have been listed on his schedule.

POLITICO’s review showed that during Obama’s first year in office, a smattering of political rallies and fundraisers showed up on the Flickr feed. But those images basically disappeared after 2010, apparently out of concern that spending taxpayer funds to promote those events could violate the Hatch Act, which regulates electioneering by government officials.

White House officials under Clinton and George W. Bush said the schedules formally released to reporters at that time were usually limited to open-press events and were often more limited than those the Obama White House releases. But journalists got a better feel for the president’s day because at an informal morning briefing known as a gaggle, the press secretary often flipped through the internal schedule and told reporters much of what the president was up to.

The practice of holding regular morning gaggles in the press secretary’s office dropped off during the George W. Bush administration and has essentially been abandoned by the Obama White House.

Obama aides would not comment on the record about the lack of regular gaggles, but one official said the administration believes the sessions are duplicative of the daily, on-camera White House briefings.

The administration appears to “start with the presumption … that the schedule is a private matter, unless there is a public event,” said Martha Joynt Kumar, a historian at Towson University who tracks White House interactions with the press. “Part of the surprise would be it seems to go against their general attitude that information should be public. [But] White House staff tend to be risk-averse. Why take a risk when you don’t need to do so?”